given in terms of the acoustic velocity potential \(\psi =\psi (x,t)\) ; see [1]}, [2]} for its derivation. The coefficient \(c>0\)  stands for the speed of sound in the fluid, whereas the ratio \(B/A\)  arises from the equation of state, which relates variations of the acoustic pressure to the ones of medium density. The dissipative mechanisms due to the viscosity and thermal conductivity of the medium are accounted for via the coefficient \(b\) , known as the sound diffusivity; cf. [3]}.
